1. Field of the Invention
This invention relates in general to amplifier circuits and more specifically to amplifier circuits with DC bias circuits.
2. Description of the Related Art
Amplifiers (e.g. radio frequency (RF) amplifiers) are utilized by devices (e.g. transmitters) to amplify a signal. FIG. 1 shows an example of a prior art amplifier circuit 101 that may be located in a transmitter (not shown). Circuit 101 is located on a printed circuit board (not shown). Circuit 101 includes an amplifier 103, an input terminal 105, and an output terminal 107. Input impedance matching circuit 109 is located between input terminal 105 and amplifier 103. An output signal path 104 couples the output of amplifier 103 to output terminal 107. Path 104 includes transmission line 123, capacitor 137, and transmission line 128 which are sized to transform the impedance of the output of amplifier 103 to the impedance output terminal 107 (e.g. 50 Ohms) at a carrier frequency of the signal (e.g. 2 GHz). Circuit 101 includes a DC bias feed path 102 coupling the output of amplifier 103 to voltage supply terminal 112 (at voltage VDD). The DC bias feed path includes λ/4 transmission line 113, decoupling capacitor 111, and low frequency decoupling capacitor 121.
Circuit 101 receives at input terminal 105 a signal and provides at output terminal 107 an amplified signal. The signal is modulated at a carrier frequency. The decoupling capacitor 111 acts as a short at the carrier frequency. λ/4 transmission line 113 transforms the short of capacitor 111 to an open at the output of amplifier 103. Thus, essentially no current at the carrier frequency propagates to voltage supply terminal 112.
At frequencies below the carrier frequency (e.g. 100 MHz and below), the DC bias feed path 102 should appear as a low impedance line such that bias current at those frequencies propagates rather unimpeded to amplifier 103 from voltage supply terminal 112.
A DC bias feed path with a low impedance line provides for lower inter-modulation distortion products and thus reduces memory effect.
One problem that may occur with circuit 101 is that the inter-modulation distortion products start to increase at frequencies above 20 MHz due to the increasing impedances of the DC bias feed path 102 at those frequencies. Such an increase in inter-modulation distortion products at those frequencies (e.g. 100 MHz and below) may be undesirable for meeting the transmission mask requirements of governing bodies (e.g. Federal Communications Commission in the United States).
What is needed is an improved amplifier circuit.
The use of the same reference symbols in different drawings indicates identical items unless otherwise noted.